Marion County, then called Champoick, was created in 1843 by the Provisional Government Legislative Committee 16 years before Oregon gained statehood on February 14, 1859. In 1849 the name was changed to Marion honoring American Revolutionary General Francis Marion.

The county is located in the heart of the Willamette Valley, and the Willamette River was established as its western boundary in 1856. Salem, the county seat and the state capital, is one of the valley's oldest cities. Among its public buildings are the Courthouse, Courthouse Square, State Capitol, Capitol Mall buildings and Salem Civic Center. The county was presided over by the Marion County Court until January 1, 1963, when the court was abolished and replaced by a Board of Commissioners.